The present invention relates to a solid oxide electrolyte fuel cell and, more particularly, an improved sealing means for solid oxide electrolyte fuel cells of the kind wherein planar fuel cells are arranged in a stack and connected in series by gas separators each serving as an interconnector.
Solid oxide electrolyte fuel cells are now a next target of research and development for practical applications, following first and second generation fuel cells, i.e., phosphoric acid fuel cells and molten carbonate fuel cells. A basic solid oxide fuel cell consists of three layers, i.e., a porous fuel electrode, a gas tight solid electrolyte and a porous air electrode. The solid oxide fuel cells have advantages that a loss of the electrolyte as found in the fuel cells of the prior art does not takes place as all the cell components are made of a solid material, and that high conversion efficiency (electrical output/heat content of fuel) is expected because of their high operating temperature of about 1000.degree. C.
For practical applications, however, they have various problems awaiting a solution. For example, difficulties are encountered in the development of cell components which are stable for a long period of time at the high operating temperatures of the cell. Other problems are found in formation of electrodes on the solid electrolyte, and in gas sealing means to prevent gas leakage between the cell components. Especially, it is required to develop a new sealing means to prevent gas leakage since the wet sealing means employed in the first and second generation fell cells can not be applied to the solid oxide fuel cells.
U.S. Pat. No. 4,799,936 discloses a method for forming monolithic solid oxide fuel cell. In this method, a solid oxide fuel cell composed of corrugated triplex layers and interconnectors are formed in the green state, and then heated by a combination of microwave and conventional heating to form a monolithic body. Thus, there is no need to provide sealing means between the fuel cells and interconnectors because of a monolithic structure of the solid oxide fuel cell module. However, such a method cannot be applied to solid oxide fuel cells comprising cell components of a metal or alloy.
As a gas sealing means for planar solid oxide fuel cells, it is considered to be effective to apply a ceramic binder to contacting surfaces of the fuel cell and gas separators. However, if the fuel cells and gas separators are joined completely by the ceramic binder to form a fuel cell stack, distortion would be produced at their junction during rising or lowering of the temperature because of difference in thermal expansion coefficient between them, resulting in cracking of the electrolyte. Also, the adhesive property of the ceramic binder is lowered by thermal cycles, resulting in leakage of the gas.